How hard it is depends on your level of mechanical aptitude and experience. It's not terribly technical but requires a certain amount of lifting, hammering, and reaching and tightening.
The old bolts are striated and will leave some grooves in the hangers. When you install the new bolts try to line up the striations on the new bolts with the grooves if possible. Be sure to drive the bolts in and not try to use the nuts to draw the bolts in.
I did our triple axle toyhauler myself over the period of a couple days. You'll need supports to hold it when you jack up the axles and take the wheels off. My advice is to leave it hooked up to the truck for support and to make sure it doesn't move any.
There's more tips and tricks and other people will be along to share theirs.